If an Artwork's Availability is set to Sold, the FASO Shopping Cart will detect that fact - whether the PayPal IPN (Instant Payment Notifications) automatically marked it as Sold or you did so manually.
This is in case 2 separate buyers have the same item in their cart. Whoever pays first purchases the item. Then customer #2 will get a screen like this when they try to check out:
If a piece is sold, existing incomplete shopping carts will basically remove that item from the cart, with the [NO LONGER AVAILABLE] message.